Generally, the screen forming process of a color picture tube is to form a three-color phosphor layer, comprising the steps of black coating and screen coating.
The black coating is to spread a non-reflective black paint on the inside of the screen panel to absorb unwanted light emission caused by scattered electrons generated between phosphor stripes or external light in order to enhance the contrast, while the screen coating is to spread the phosphors of three colors (Green, Blue, Red) in the holes between the black stripes formed by the black coating.
In order to form the phosphor layer, after a plurality of light-absorbing black stripes 6 are formed on the inside of the screen panel 2 as shown in FIG. 2, a green phosphor slurry comprising a phosphor powder, salt of chromic acid that is light-sensitive, polyvinyl alcohol and surfactant is loaded on the inside of the screen panel 2 rotated with a high speed to spread the phosphor slurry over the whole surface of the panel.
The phosphor slurry layer on the inside of the panel is dried, and then exposed to light developing into the holes of green, blue and red. If the panel and shadow mask are connected and exposed to an infrared light source 4 as shown in FIG. 3, the portions of the phosphor layer illuminated with the infrared ray become insoluble by light coupling reaction between the polyvinyl alcohol (PVA) and salt of chromic acid firmly adhered to the panel. Further the outer side of the panel is exposed to a light source 5 in order to enhance the effect of the infrared light source 4. Likewise the blue and red phosphor layers are sequentially formed.
In this conventional method of forming the phosphor layer on the screen panel, when the panel 2 is rotated with a high speed to spread the phosphor slurry over the whole surface, the central portion of the panel 2 hardly receives the centrifugal force, and the phosphor slurry disposed in the corners is hindered from spreading by the skirt portions 2a. Consequently, the phosphor layer 1 comes to have thicker portions in the central and corner regions than in the other regions, as shown in FIG. 1.
The thicker central and corner portions of the phosphor layer 1 do not receive the light of the infrared light source 4 enough to influence the depth, so that the phosphor layer 1 may be separated during development. Furthermore, in order to resolve this problem if the light intensity of the infrared light source is increased, the relatively thinned portions of the phosphor layer are excessively exposed to the light, and the width of the stripe 7 is widened to the positions of the phosphor layers of different colors so as to impair the clearness of the screen colors of images. In addition, the light source 5 such as incandescent lamp disposed in front of the central portion of the outer side of the panel 2 may prevent the separation of the central portion of the phosphor layer, but there is not resolved the problem caused by inadequacy of the exposing light in the corner portions.